The present invention relates to pitches, in particular to coal tar pitches, which are intended to be employed as agents for impregnating carbon or graphite components to increase the density and mechanical strength of these components. In this specification, these pitches will be referred to by the term "impregnation pitch".
In general, pitches are characterized by various analytical data, the knowledge of which enables their utility value for the chosen application to be assessed. A list of the various characteristics of pitches, which will be referred to in this case, is given below:
(1) The content of substances which are insoluble in quinoline (QI), and the content of substances insoluble in toluene (TI) which are expressed in percent by weight, are obtained by the operating procedures given in the standards ISO 6791 and ISO 6376 respectively;
(2) A Kraemer-Sarnow softening point (KS), which is expressed in .degree.C., is determined according to the mercury procedure specified in DIN standard 52025;
(3)The fixed carbon content (FC), which is expressed in percent by weight, reflects the coke yield or the carbon value of the pitch and is calculated according to the method described in ISO standard 6998.
A pitch which is considered to be satisfactory to form an impregnation pitch should meet the following specifications:
(1) a KS softening point ranging from about 60.degree. C. to 80.degree. C.;
(2) a content of quinoline-insoluble substances (QI) which is generally below or equal to 7% by weight; and
(3) content (FC) of at least 46%; for example, of between 46% and 54% by weight.
However, the essential criterion of success of an impregnation pitch consists of another measurement, namely, the rate of filtration or filterability index. This criterion is characteristic of coal tar pitches because it reflects the impregnating nature of the pitch. The filterability indexes have been determined in this case by the method described below. In a filtration cell, identical to that described by J. W. Clarke, T. D. Rantell in the periodical "Fuel", 1980, 59, 35-41, a sample of about 50 g of the pitch to be analyzed, which is heated beforehand in an oven, is subjected to filtration through a metal centered disc at a constant pressure of 5.times.10.sup.5 Pa and at a constant temperature of 225.degree. C. The curve m=f(t) is recorded (where m is the mass of the filtered pitch expressed in grams and t is the time expressed in minutes), and the filterability index (I) is calculated according to the formula: ##EQU1## wherein
m.sub.1 is mass collected at time t.sub.1 ; and
m.sub.2 is mass collected at time t.sub.2.
Until now, it was thought by a person skilled in the art that the thermal maturation treatment of an impregnation pitch had no influence on, or was even disadvantageous to, the rate of filtration.